1. Field of the Invention
The invention concerns line transfer photosensitive matrices, namely matrices of photosensitive elements arranged in networks of lines and columns, wherein each line of photosensitive elements can be addressed so as to discharge the charges, generated by the photosensitive element located at the intersection between this line and this column, on to a column conductor of a defined column of the matrix. The charges thus discharged are applied to a compartment of a charge transfer shift register, the other compartments of which receive the charges discharged by the other columns of the matrix.
The matrix works in two stages which are periodically renewed: a first stage during which the photosensitive elements produce and store charges under the effect of an illumination, while the shift register simultaneously shifts, towards its output, those charges that it has received during the previous period; and a second stage during which the register receives, from the columns, those charges collected by the photosensitive elements during the first stage.
The invention more especially concerns the read circuit of this matrix, namely the circuit through which the charges are transferred from the columns to the register during the second stage of each period.
2. Description of the Prior Art
One of the problems encountered with this type of matrix is that of the noise introduced during the transfer of the charges, through the read circuit, between the columns of the matrix and the shift register. This noise is generally of thermal origin. It consists of a certain quantity of undesirable charges, which are not caused by the illumination of the photosensitive elements but are transferred at the same time as the charges generated by the illumination.
Now, the higher the capacitances applied, the greater is the noise. In particular, given the matrix structure of the device, the overall capacitance of a column of photosensitive elements is the sum of the capacitances of the individual, photosensitive elements connected to this column. For example, the individual capacitance of a photosensitive element may be typically one picofarad and that of a column may be 2000 picofarads for a large-format matrix of 2000 lines based on amorphous silicon.
It may be considered that the B.sup.th thermal noise introduced when there is a capacitance with a value C is substantially equal to: EQU B.sup.th =(kTC/q.sup.2).sup.1/2.
Where K is Boltzmann's constant, T is the temperature in degrees Kelvin and q is the charge of the electron.
In concrete terms, for a capacitance of 1 picofarad, the noise is equal to 400 electrons which disturbs the useful signal transferred to the column. For a column capacitance of 2000 picofarads, the noise is 18,000 electrons. This is a huge value if it is considered that it is sought to detect a signal corresponding to a few thousands of electrons for low illuminations.
To reduce the noise introduced by the column capacitance and transferred from the column to the output shift register, it has already been proposed to insert a feedback amplifier between the column and a transfer gate used for the periodic removal of the charges of the column towards the shift register. The amplifier has a negative gain -G. Thus, a positive potential fluctuation at the column, corresponding to an excess of transferred noise charge, means a reduction in the potential applied to the transfer gate: this reduces the injection current and consequently causes the transfer of a smaller charge. The amplifier thus introduces a compensation effect which restricts the injected noise charge. It may be then assumed everything happens as if the transfer beneath the gate has come from a column with a capacitance C/G instead of C. The thermal noise introduced is therefore divided by G.sup.1/2.
A great deal can be gained, in this way, on the thermal noise and, therefore, on the dynamic range of the useful signal that can be obtained on the side of the low levels of illumination.
Unfortunately, the gain -G amplifier, which has been introduced into the circuit, itself introduces noise into the transferred charges.